This invention relates to a tire pressure sensing system for on-board use in an automotive vehicle such as a car, truck or tractor-trailer to convey inflation pressure information from the rotating pneumatic tires on which the vehicle travels.
It is recognized that the only way to guard against prolonged periods of underinflation of pneumatic tires is by frequently checking pressure and replenishing the tires with air if underinflation is indicated. Visual observation of a tire as an indicator of underinflation is not reliable, especially with contemporary tire designs. The manual checking of each of the four tires on an automobile, and possibly the spare, by means of a strictly mechanical tire pressure gauge is obviously more accurate than visual observation but it involves more than a nominal expenditure of effort. For each tire, the valve cap must be removed, the tire pressure gauge pressed against the exposed valve stem to obtain a reading, the tire filled if the reading indicates too low of pressure, and finally the valve cap must be replaced. It is unrealistic to expect the typical vehicle owner to perform this degree of maintenance on a frequent basis. At best, tire pressure checking is done randomly, if at all.
The owners and operators of commercial rigs and truck fleets are affected on a much larger scale by underinflated tires than are the owners of personal automobiles. In checking a large number of tires on a truck or rig, there will doubtless be many tires which, when checked, are found to be properly inflated, and therefore the time involved in checking these tires is an extra expense to the fleet and rig owners which could have been avoided if it were known beforehand that the pressure was proper.
There are numerous patents that relate to various forms of apparatus for indicating tire pressure without uncapping the valve stem and taking a measurement with a manual tire pressure gauge. Some relate to on-board pressure measurement systems where an occupant of the vehicle is informed if a tire is underinflated. On-board systems are advantageous if they can provide information concerning the present status of the tires, thereby presenting the driver with the opportunity to detect underinflation in its incipiency, and consequently the opportunity for taking corrective action much sooner than might otherwise be the case. Many of these prior schemes are too elaborate, too complicated, or too expensive to be practical. Some give indication only when the pressure drops below a threshold and are incapable of sensing over a range of pressures.
Apart from the obvious safety implications of underinflated tires, it is well documented in the industry that underinflated tires wear at significantly higher rates than properly inflated ones. The economic loss to the public due to premature wear which accompanies underpressurized tires would likely surprise many people.
A commercially acceptable on-board tire pressure sensing system should alert the vehicle operator to an incipient underinflation condition of any tire giving him the opportunity to avoid the above-mentioned problems incidental to underinflation. The deployment of this system also would eliminate the need to manually check the individual tires in the manner described above. For the owners of personal cars and trucks this is a real convenience; for fleet and rig operators, it would afford savings from both the standpoint of reducing premature tire wear and also avoiding the time and expense involved in checking tires which are already properly pressurized.
One of the serious impediments to a successful tire pressure sensing system involves the transmission of the tire pressure information from the rotating tire and wheel. Prior proposals include the use of slip rings, radio frequency transmitters, and magnetic pick-ups. Insofar as the applicant is aware, none of these prior systems has received widespread commercial acceptance, if indeed any at all.
An example of a system that is believed capable of obtaining commercial acceptance is disclosed in the allowed commonly assigned, co-pending application of Louis Galan, Ser. No. 026,953, filed Mar. 17, 1987 and entitled "On-board Tire Pressure Indicating System". According to the disclosure of that patent application, a Hall Effect sensor is disposed on the vehicle adjacent to each rotating wheel on which a pneumatic tire is mounted. On the wheel is a transmitter which comprises three magnets. The magnets are spaced apart in a general circumferential sense on the wheel and lie on essentially the same radius. As the wheel rotates, the magnets repeatedly sweep past the Hall receiver which detects the passage of each magnet. Two of the three magnets are disposed in a predetermined fixed circumferential spacing on the wheel. The third magnet is selectively positioned in a circumferential sense relative to the other two magnets in accordance with the pressure in the tire. Hence, the distance between the two fixed magnets represents a reference measurement dimension, and the distance between the third magnet and one of the two fixed magnets represents an inflation pressure measurement dimension. The Hall sensor detects the passage of the three magnets by a time sequence of three pulses. The time between the two fixed magnet pulses corresponds to the reference measurement dimension while the time between the selectively positioned magnet pulse and one of the two fixed magnet pulses corresponds to the inflation pressure measurement dimension. The ratio of the two measurement dimensions is essentially independent of the rotational speed of the wheel and tire. The actual reference measurement dimension on the wheel is constant, but the sensed reference measurement dimension, in terms of time interval between the two fixed magnet pulses, will vary as a function of the rotational speed of the wheel and tire, specifically being inversely proportional to speed. The actual inflation measurement dimension on the wheel is constant for a given inflation pressure, and in analogous manner to the sensed reference measurement dimension, the sensed inflation measurement dimension as measured by the time between the one fixed magnet pulse, and the selectively positioned magnet pulse will be inversely proportional to rotational speed. The ratio of the sensed reference measurement dimension to the sensed inflation measurement dimension is essentially speed insensitive because velocity factors out when the two sensed measurements are ratioed.
The signals from the Hall sensor are supplied to electronic circuitry which performs the ratio calculation to provide the actual inflation pressure. Different pressures produce different positioning of the positioned magnet whereby the sensed pressure measurement dimension is correlated with pressure so that when the ratio is taken between the sensed reference measurement dimension and the sensed inflation pressure measurement dimension, the true inflation pressure measurement is attained. The inclusion of the Hall Effect sensor in combination with this three magnet system produces a very accurate and reliable means for transmitting the pressure information from the rotating wheel and tire to the electronics. The electronics can then operate an appropriate display or displays on the vehicle instrument panel for indicating the tire pressure status. Advantageously the system is capable of providing a numerical readout of the actual pressure and/or operating warning lights to indicate when a predetermined low pressure has been reached. The particular type of readout that is used in any given system is of course one of choice.
The present invention in certain respects relates to an improvement upon the system that is disclosed in allowed application Ser. No. 026,953. One aspect of the improvement resides in the ability of the system to perform the pressure sensing function with a fewer number of magnets. Specifically, there will be disclosed two embodiments of the present invention, one using only two magnets in the transmitter and the other using a single magnet and a keeper in the transmitter. A Hall Effect sensor continues to be employed as the receiver of information from the transmitter as the transmitter rotates past the receiver.
Another aspect of the invention relates to the manner in which the various component parts, particularly the magnet parts, are organized and arranged in the transmitter. This organization and arrangement produces particular magnetic pulse transmissions that are received by the Hall sensor as the transmitter rotates past. The Hall sensor is in turn connected with electronic circuitry that processes the signals to develop tire pressure information.
The electronics that is associated with the Hall sensor represents a still further aspect of the invention. The electronics comprises a unique organization and arrangement of conventionally recognized electronic circuits such as comparators, signal conditioners, phase detectors, flip-flops, etc. More specifically, the unique configuration of magnetic parts in the transmitter results in the creation of a bi-directional pulse waveform in the Hall sensor during each revolution. This bi-directional pulse waveform contains the sensed tire pressure information in a particular format and the electronics associated with the Hall sensor process the waveform in a manner which extracts the tire pressure information to produce an output signal that can drive a display device such as an indicator light and/or a meter. The meter of course could be either analogue or digital. In this way, useful tire pressure information is presented to the operator of the vehicle while the vehicle is in motion .